The head of Germany's foreign intelligence service was in Ukraine when Russia invaded and had to be taken home overland in a gruelling two-day journey by special forces when the country's airspace was closed, Focus magazine reported.

The Federal Intelligence Service (BND) confirmed that Bruno Kahl had been in Ukraine on Wednesday for "urgent talks" but had to change his departure plans after the invasion began.

"Because of the outbreak of hostilities and the closure of Ukraine's airspace, the President had taken the land route back," the BND said in a statement on Friday, which did not mention the part special forces had allegedly played.

"This was a difficult and lengthy journey because of the streams of refugees going in the same direction. He is back in the European Union and is expected in Berlin today."

Bild newspaper quoted sources as saying that Kahl had been in Ukraine to pass on important information in person. Kahl, a former politician rather than a career secret agent, had missed an earlier evacuation of German diplomats. He was now in Poland.

Massive flows of refugees fleeing the advancing Russian army have resulted in queues of many hours at Ukraine's borders with its western neighbours.